Relationship between Structural Properties and CO2 Capture

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Ind. Eng. Chem. Res. 2008, 47, 6216–6220

Relationship between Structural Properties and CO2 Capture Performance of CaO-Based Sorbents Obtained from Different Organometallic Precursors Hong Lu, Ataullah Khan, and Panagiotis G. Smirniotis* Chemical and Materials Engineering Department, UniVersity of Cincinnati, Ohio 45221-0012

A series of CaO-based sorbents were synthesized from various organometallic precursors, namely, calcium propionate, calcium acetate, calcium acetylacetonate, calcium oxalate, and calcium 2-ethylhexanoate, by a simple calcination technique. In general, the five organometallic precursors (OMPs) exhibit a three-step weight loss regime in their respective thermogravimetic (TG) profiles. The first weight loss occurs because of dehydration in the temperature range of 50-200 °C. The second one results from decomposition leading to the formation of calcium carbonate in the temperature range of 450-550 °C. The calcium carbonate so formed then undergoes decarboxylation at higher temperatures of 710-750 °C and results in the formation of calcium oxide. Among the various precursors evaluated, CaO-sorbents obtained from calcium propionate and calcium acetate precursors were found to exhibit the highest CO2 capture capacity. The observed results were correlated with the intrinsic properties of the precursors by means of various techniques like thermogravimetric analysis (TGA), pore-size distribution (PSD), and differential scanning calorimetry (DSC). It was found that these two sorbents possessed higher surface area and larger pore volume compared to other sorbents prepared in this work. Thermal decomposition of these two OMPs resulted in the maximum evolution of heat, which could eventually lead to the generation of larger macropores, thus explaining the resultant CO2-uptake capacity we observed. Interestingly, the CO2 capture capacity of the sorbents was found to be directly proportional to the porosity per unit surface area. In summary, we were successful in correlating the intrinsic properties of an OMP to the eventual CO2 capture capacity of the sorbent. From the present investigation, it seems that the amount of heat evolved during the course of decomposition plays a direct role in the resultant porosity and thereby regulates the eventual CO2 capture capacity. 1. Introduction Carbon dioxide is one of the main greenhouse gases contributing to global warming. Current commercial amine based processes capturing CO2 are only applicable on a small scale in limited plants.1,2 Moreover, the operation temperature of this technology is usually between 50-140 °C, which largely restricts its application in large-scale processes requiring CO2 capture because of cooling cost. Sorbents based on calcium oxide are well-known for their capability to capture CO2 at high temperatures, thereby forming calcium carbonate, which upon thermal decomposition regenerates CaO. The carbonation reaction is exothermic, while the decarbonation (calcination) reaction is endothermic. The reversible carbonation and decarbonation of calcium oxide with CO2 form a cyclic process, therefore providing great potential for capturing CO2 at higher operation temperature between 500 and 900 °C.3–7 The use of calcium oxide based sorbents to capture CO2 is associated with issues such as how fast and to what extent carbonation and decarbonation take place and how the sorbents progress in extended cycles. Researchers have tried to enhance sorbent performance by various means. It is well-established that the CO2 capture capacity of the sorbents typically goes down with increasing number of cycles. Numerous studies have been undertaken to improve the performance of CO2 sorbents: Aihara et al.4 promoted CaO with calcium titanate, while Gupta and Fan5 managed to enhance performance of CaO sorbents by synthesizing them from precipitated calcium carbonate. Other researchers8,9 enhanced the reactivity and reversibility of CaO sorbents by an intermediate hydration treatment step between multicyclic CO2 adsorption. Reddy and Smirniotis6 enhanced * Corresponding author. Tel.: +1 513 556 1474. Fax: +1 513 556 3473. E-mail: [email protected] (P.G. Smirniotis).

the surface basicity of sorbents by doping alkali metal Cs onto CaO, thereby improving performance. Li et al.10 reported that Ca12A14O33 could enhance sorbents reversibility by acting as a binder, retarding the sintering of CaO particles. Studies have also shown that the morphology of the calcined sorbent relates with the calcination conditions, with higher temperatures and longer calcination times leading to greater losses in surface area and performance.11 Typically, the most common sources for obtaining CaO have been CaCO3 (limestone). However, to the best of our knowledge, studies on CaO sorbents obtained from various organometallic precursors (OMPs) like propionate, acetylacetonate, oxalate, ethylhexanoate, etc. are scarce in the literature. These high-value precursors would generate highperformance CaO sorbents for specialty applications. In this article, we report the preparation of CaO powders from various OMPs by the simple calcination method. The morphology and structural characteristics of the powders were studied. The carbonation-decarbonation properties of the powders were evaluated in detail. An effort has been made to correlate the structural characteristics of the sorbents with their eventual sorption-desorption performance and to study the role played by the OMPs on the resultant performance of the individual sorbent. 2. Experimental Section 2.1. Sorbent Synthesis. The various OMPs chosen in the present study are as follows: calcium propionate (95%), calcium acetate (certified), calcium acetylacetonate hydrate (99.95%), calcium oxalate (99.999%), and calcium 2-ethylhexanoate (98%). With the only exception of calcium acetate (Fischer), all other precursors are of Aldrich make. The sorbents were synthesized by calcining each precursor in a Lindberg Blue M furnace, in which the precursor was heated from ambient to 900 °C at a rate of 10

10.1021/ie8002182 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

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°C/min and then was kept at 900 °C for 1 h in air environment. Furnace temperatures were calibrated by using an external K-type thermocouple. 2.2. Sorbent Characterizations. 2.2.1. Surface Area andPore-SizeDistributionAnalysis.TheBrunauer-Emmett-Teller (BET) surface areas (SAs) were obtained by N2 adsorption on a Micromeritics Gemini 2360 instrument. Prior to analysis, samples were oven-dried at 120 °C for 12 h and flushed with argon for 2 h. The pore-size distribution analyses were conducted by N2 physisorption at liquid N2 temperature using a Micromeritics ASAP 2010 apparatus. All samples were degassed at 300 °C under vacuum before analysis. 2.2.2. X-ray Diffraction Measurements. Powder X-ray diffraction (XRD) patterns were recorded on a Phillips Xpert diffractometer using a nickel-filtered Cu KR (0.154056 nm) radiation source. The intensity data were collected over a 2θ range of 5-80° with a 0.02° step size and using a counting time of 1 s/point. Crystalline phases were identified by comparison with the reference data from ICDD files. 2.2.3. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) studies were performed on a differential scanning calorimeter 2010 from TA instruments, Inc. All experiments were conducted under air atmosphere with a flow rate of 30 sccm. Approximately 5-10 mg of OMPs were employed, and the temperature was ramped up to 600 °C at a rate of 10 °C/min. Thermal Advantage version 1.1A software was employed for the post-measurement analyses. 2.2.4. Evolved Gas Analysis. Evolved gas analyses were performed using a MKS Cirrus LM90 RGA unit (residual gas analyzer). The specific OMPs were subjected to thermal decomposition in a thermogravimetric analyzer (TGA) apparatus (the temperature was ramped from ambient to 750 °C at a 10 °C/min heating rate), and the gas thus evolved from the sample was sent to the RGA unit for analysis. The mass spectrometry (MS) plots of intensity vs m/z were recorded as a function of time and temperature. 2.3. Sorbent Chemisorption. The carbonation and decarbonation (calcination) experiments were conducted with a Perkin-Elmer Pyris-1 TGA. A small amount of sorbent (4-8 mg) was used for carbonation of CO2 at high temperatures. During the carbonation process, 20 mL/min of CO2 reactant gas along with 45 mL/min of helium purge gas was passed over the sample pan. During the decarbonation, 20 mL/min of CO2 was replaced by 20 mL/min of helium in order to keep the total gas flow constant at 65 mL/min. In this work, typical carbonation time was set at 300 min unless otherwise specified to get relatively high isotherm equilibrium and decarbonation time was set at 30 min to allow the sorbent to decarbonate completely. During the entire experiment, both sorbent weight and temperature were continuously recorded as a function of time with a rate of 1 reading per second.

Figure 1. TGA plots of decomposition of various OMPs.

process as analyzed by a residual gas analyzer (RGA) and the % weight lost in each consecutive decomposition step from each OMP are presented in Table 1. Figure 2 shows the derivative of the TG curve, or the differential thermogravimetric (DTG) curve, which is often useful in revealing additional details. The DTG curve is sometimes used to determine inflection points on the TG curve, to provide reference points for weight change measurements in systems where the weight losses are not completely resolved. The TG data of calcium propionate revealed 71% loss of weight in three consecutive steps. Similarly, in the case of calcium acetate, 66.5% weight loss took place, while 78.4% weight loss was observed for the calcium acetylacetonate. For calcium oxalate and calcium 2-ethyl hexanoate precursors, weight losses of 59% and 61%, respectively, were noted between ambient and 750 °C, above which no substantial weight loss could be noted. This indicates that, over 750 °C, the sorbents are quite stable in terms of phases and chemical composition. The measured weight losses (as listed in Table 1) agree well with the theoretical losses, expected in accordance with the decomposition pattern as shown below:

3. Results and Discussion The various organometallic precursors employed to prepare calcium oxide sorbents in the present study were subjected to thermogravimetric (TG) analysis. The obtained thermograms, between 50 and 750 °C, are shown in Figure 1. In general, the TG profiles of all five OMPs exhibit a three-step weight loss regime. The first weight loss occurs on account of dehydration and is observed in the temperature range of 50-200 °C. The second decomposition in the temperature range of 450-550 °C leads to the formation of calcium carbonate. The calcium carbonate so formed undergoes decarboxylation at higher temperatures of 710-750 °C and forms calcium oxide consequently. The various species evolved during the decomposition

Interestingly, one could also see the direct correlation between weight loss and mass spectroscopy (MS) signals as recorded by the RGA/MS unit. As noted from Table 1, the RGA/MS results show detectable quantities of decomposed productssH2O (m/z ) 18), CO (m/z ) 28), and CO2 (m/z ) 44). Decarbonylation (loss

6218 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 1. Decomposition Steps of Various Precursors As a Function of Temperature organometallic precursor calcium propionate-Ca(C2H5COO)2 calcium acetate-Ca(CH3COO)2 calcium acetylacetonate-Ca(CH3COCHCOCH3)2 calcium oxalate-Ca(COO)2 calcium 2-ethylhexanoate-Ca(C7H15COO)2

parametric studies

dehydration

decomposition to carbonate

decomposition to oxide

temperature % wt loss m/z species temperature % wt loss m/z species temperature % wt loss m/z species temperature % wt loss m/z species temperature % wt loss m/z species

90 °C 4 18 150-200 °C 5 18 190 °C 6 18 130 °C 5 18 90 °C 3 18

480 °C 44 18, 28, and 44 440 °C 35 18, 28, and 44 560 °C 55 18, 28, and 44 510 °C 21 28 510 53 18, 28, and 44

740 °C 23 44 740 °C 26 44 740 °C 17 44 740 °C 33 44 740 °C 28 44

of CO) was observed in the decomposition of calcium oxalate to calcium carbonate. All the precursors exhibit a characteristic m/z peak at 44 (due to CO2 evolution) during the decomposition of carbonate to oxide, beyond which no detectable species were observed in the RGA/MS analysis, in complete agreement with the TG analysis report. Table 2 represents the BET surface area and pore volume values of various CaO sorbents investigated in the present study. As can be noted from Table 2, CaO sorbents obtained from different OMPs (by simple calcination) exhibit variable surface areas and porosities. The XRD measurements revealed the existence of crystalline CaO structure for all the sorbents, as presented in Figure 3.

Figure 2. DTG plots of CaO sorbents made from various OMPs.

Carbonation of calcium oxide has been widely studied. It is widely accepted that the gas-solid reaction of CO2-CaO proceeds via two stages.12,13 In the first stage, a rapid heterogeneous chemical reaction takes place. In the second stage, the reaction slows down and is rate-controlled by the diffusion through a layer of CaCO3. This produced layer of CaCO3 prevents the exposure of unreacted CaO located in the particle core for further carbonation with CO2. Theoretically, 56 g of CaO should react with 44 g of CO2 to form 100 g of CaCO3. In other words, this amounts to 0.786 (g of CO2)/(g of CaO). However, structural limitations prevent the attainment of theoretical conversion. The limitation on total conversion is a direct result of the initial pore-size distribution of the CaO sorbent. Microporous sorbents are vulnerable to pore blockage and plugging because of the formation of a larger CaCO3 [molar volume of CaCO3 (37 cc mol-1) > CaO (17 cc mol-1)], preventing the access of CO2 to the inner pore surface. The reactivity and CO2 capture capacity of these CaO sorbents were evaluated by measuring the increase in weight as a function of time on a TG analyzer. The increase in weight is caused because of the carbonation of CaO, leading to the formation of CaCO3. The results of the CO2 capture capacity conducted at 700 °C are presented in Figure 4. As observed from Figure 4, the sorbents obtained from calcium propionate [Ca(C2H5COO)2] and calcium acetate [Ca(CH3COO)2] exhibit excellent CO2 capture capacity. Within the first 15 min of carbonation, a steep increase from zero to 82% and 60% was observed in the cases of these sorbents, respectively. A maximum of 95.5% and 90% carbonation could be observed for extended periods of time over these two sorbents, respectively. Among all the sorbents examined in the present investigation, CaO derived from calcium propionate exhibits the highest CO2 capture capacity, followed by CaO derived from calcium acetate and calcium acetylacetonate precursors. On the other hand, CaO sorbents, obtained from calcium oxalate and calcium 2-ethylhexanoate, displayed very poor CO2 capture capacity. On the basis of the above findings, one can assume that the differences in the reactivities arise because of the differences in the sorbent morphology and not by the chemistry of the gas-solid reaction that takes place on the CaO surface, as all the CaO sorbents (obtained from different

Table 2. Physisorption Measurements over Various CaO-Sorbents Derived from Different Organometallic Precursors physisorption measurements organometallic precursor derived-CaO sorbent calcium calcium calcium calcium calcium

propionate-CaO acetate-CaO acetylacetonate-CaO oxalate-CaO 2-ethylhexanoate-CaO

BET SA (m2 g-1)

pore volume (cm3 g-1)

pore volume/BET SA (10-8 m)

15 20 12 5.9 9.3

0.18 0.22 0.09 0.02 0.015

1.2 1.1 0.75 0.34 0.16

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OMPs) exhibit exactly similar crystal structure (Figure 3). In order to get further insight into relationship between the “CO2 capture capacity” and “sorbent morphology”, the sorbents were further characterized by differential scanning calorimetry and pore-size distribution (PSD) studies. Accordingly, PSD studies were performed on the various CaO sorbents to evaluate the surface morphology of the samples; the results are presented in Figure 5. Interestingly, a significantly diverse pore-size distribution was observed among the various sorbents investigated. Macroporous structures were noted in sorbents obtained from calcium propionate and calcium acetate. More specifically, pores in the size ranges of 80-230 nm and

40-125 nm were observed, respectively. The above results suggest that large-size pores (macropores) would be less susceptible to pore blockage and, thus, provide higher CO2 capture capacity even with lower total surface area, as observed in this study (Table 2). These structures would also provide sufficiently available surface area to ensure rapid kinetics. To elucidate the main cause for the appearance of this drastically different porosity in each CaO sorbent obtained from different precursors, extensive DSC studies were performed and the results obtained are presented in Figure 6. The heat evolved during the course of thermal decomposition, also termed as “heat of combustion”, is characteristic of a given OMP. Heats of combustion are also used in comparing the stabilities of chemical compounds: the one releasing more energy (i.e., with the higher heat of combustion) is the less stable one, since it was more energetic in its compounded form. It is observed that all the precursors follow the same course of decomposition (eq 1). However, the degree of heat evolved during the process varied from precursor to precursor. The precursor, which releases more heat during the course of thermal decomposition, forms a more porous structure. As noted from Figure 6, all the OMPs analyzed in the present study exhibit prominent exothermic peak(s) beyond 275 °C. Interestingly, the heat evolved in the temperature

Figure 3. XRD patterns of CaO sorbents made from various OMPs [* ) CaO (PDF-ICDD: 44-0777)].

Figure 4. Carbonation studies over CaO sorbents made from various OMPs.

Figure 5. Pore-size distribution of CaO sorbents made from various OMPs.

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area and large pore volume of the sorbent. Calcium propionate and calcium acetate decompose with the maximum evolution of heat, which eventually leads to the generation of larger macropores, thus explaining the resultant CO2-uptake capacity. On the contrary, the poor performance of CaO sorbents obtained from calcium oxalate and calcium 2-ethylhexanoate precursors is due to their microporous morphology. During the initial course of carbonation, a layer of CaCO3 is formed over the surface of the sorbent. Thus, CO2 has to diffuse into the unreacted sorbent interior through the newly formed solid CaCO3 layer. This CaCO3 layer eventually blocks the micropores because of the large volume of CaCO3 as compared to CaO. The diffusion resistance in solid is much higher than that under gas phase. It should be noticed that no chemical modification was done to the calcium propionate-CaO and calcium acetate-CaO to obtain higher CO2 capture capacity. This offers a great advantage for specialty applications because of the simplicity and the cost effectiveness of the synthesis, the high CO2-uptake capacity, and the rapid kinetics observed. Acknowledgment We are grateful to the U.S. Department of Energy for financial support under the grant DE-FG26-03NT41810. Literature Cited

Figure 6. DSC plots of CaO sorbents made from various OMPs.

range of 375-525 °C, i.e., during the course of CaCO3 formation, seems to play a vital role on the resultant sorbent morphology observed. We were able to deduce a direct oneto-one relationship between the degree of exothermicity observed during the course of CaCO3 formation (375-525 °C) and the sorbent pore morphology. It is observed that, among the five OMPs studied in the present work, calcium propionate decomposition resulted in the maximum evolution of heat (3830 J/g), followed by calcium acetate (2490 J/g). These two sorbents possessed higher surface area and larger pore volume compared to other sorbents prepared in this work (Table 2). The remaining three sorbents, calcium acetylacetonate, calcium oxalate, and calcium 2-ethylhexanoate, during the course of decomposition to CaCO3 form (375-525 °C), released 2160, 340, and 183 J/g of heat, respectively. From the above observation, it seems that there is a direct correlation between the heat evolved (degree of exothermicity) and the size of the pores formed: greater exothermicity yields larger pores. Interestingly, the CO2 capture capacity of the sorbents was found to be directly proportional to the porosity per unit surface area, as calculated in Table 2. In summary, we were successful in correlating the intrinsic properties of an OMP to the eventual CO2 capture capacity of the sorbent. From the present investigation, it seems that the amount of heat evolved during the course of decomposition plays a direct role on the resultant porosity and indirectly regulates the eventual CO2 capture capacity. 4. Conclusions Calcium oxide sorbents synthesized from calcium propionate and calcium acetate exhibit high CO2 uptake capacity among the five OMPs originated sorbents studied in the present work. The enhanced performance can be attributed to the high surface

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ReceiVed for reView February 6, 2008 ReVised manuscript receiVed April 17, 2008 Accepted April 29, 2008 IE8002182